CHLORINE-FREE BLEACHING OF CHEMICAL PULP INVOLVING A TWO STAGE TREATMENT WITH CHELATING AGENT

Abstract: A method for bleaching lignocellulose-containing fiber material in the form of chemical pulp without using chloric bleaching agents where the pulp is treated with chelating agent and is bleached with ozone and peroxide. Immediately after the ozone bleaching the pulp is treated with chelating agent in a second chelating step at a pulp concentration of 1-10 % for 1-30 minutes at the temperature of 20-90 DEG C and the pH value 1,5-5. The pulp thus treated is washed and dewatered to a concentration of 8-40 % prior to the peroxide bleaching.

Description: Chlorine-free bleaching of chemical pulp involving a two stage treatment with chelating agent This invention relates to the bleaching of lignocellulose- -containing fiber material in the form of chemical pulp without using chloric bleaching agents. The invention, more precisely, relates to ozone bleaching of the pulp and sub¬ sequent peroxide bleaching. For rendering ozone bleaching of the pulp to take place more selectively, the pulp is first treated with a chelating agent, for example DTPA or EDTA. Hereby metal ions are solved out of the pulp and are removed by dewatering the pulp prior to the ozone bleaching. The pulp bleached with ozone still contains metals, which partially are absorbed on the fibers and partially solved in surrounding liquid. These metals must be removed for being able to bleach with peroxide in a subsequent step. According to a conventional method, a treatment with chelat¬ ing agent is carried out at a pulp concentration of 1-10% for 1-60 minutes at a temperature of 20-70°C and a pH value of 2-5. The pulp is thereafter dewatered and bleached with ozone. After the ozone bleaching the pulp is washed and dewatered, whereafter a peroxide bleaching takes place. Alternatively, an additional treatment with chelating agent is carried out after the washing and de¬ watering. This treatment with chelating agent usually is carried out for 30-120 minutes at 60-90°C. The pulp must thereafter again be washed and dewatered prior to the peroxide bleaching. According to the firstmentioned one of the above known methods, no satisfactory ISO-brightness is obtained, and according to the alternative method extra treatment steps are required which complicate the process and make it more expensive. According to the present invention, these problems are elim¬ inated in that the method is relati ely simple and at the same time yields a high ISO brightness of the pulp. The characterizing features of the invention are apparent from the attached claims. The invention is described in greater detail in the follow¬ ing with reference to the accompanying drawing, in which Fig. 1 is an example of a flow chart for a method accord¬ ing to the invention, and Fig. 2 is a diagram showing the brightness as a function of the hydroperoxide charge, for known methods and, resp. , for a method according to the invention. According to the flow chart in Fig. 1, chemical pulp first is fed to a first chelating step 10 for treatment with EDTA or DTPA. The pulp concentration should be 1-10% and the treatment be carried out for 1-60 minutes, preferably 5-30 minutes. The temperature should be 20-70°C, preferably 20-40°C, and the pH value 2-5. After the first chelating step 10 the pulp is washed and dewatered, for example in a washing press 11, for the removal of solved metal ions. The pulp is directed therefrom to an ozone step 12 for ozone bleaching. Immediately after the ozone step 12 the pulp is diluted to a concentration of 1-10% and fed to a second chelating step 13. There it is treated with chelating agent, preferably DTPA or EDTA, for 1-30 minutes, preferably 5-20 minutes, at the temperature 20-90°C, preferably 40-80°C and the pH value 1,5-5. The amount of charged DTPA or EDTA preferably can be up to 0,5 kg/ton pulp. After this second chelating step 13 washing and dewatering is carried out to a concentration of 8-40% in a second washing step 14, whereafter the pulp is directed to a peroxide step 15 for peroxide bleaching. According to this method, the pulp can be bleached to a brightness exceeding 83% ISO. In the diagram in Fig. 2 the brightness is shown as a funct¬ ion of the peroxide charged at different types of treatment between ozone and peroxide step. Curve 1 relates to a conventional method with peroxide bleaching after the ozone-bleached pulp is only washed and dewatered . Curve 2 relates to peroxide bleaching of ozone-bleached pulp, which in known manner had been washed, dewatered, treated with chelating agent and again be washed and dewater¬ ed prior to the peroxide bleaching. Curve 3 relates to a method according to the invention. It appears from Fig. 2 that the bleaching according to the invention (Curve 3) yields a considerably better result than conventional bleaching according to Curve 1. It further appears that the bleaching according to the invention yields a result which substantially corresponds to the result of the bleaching according to Curve 3, in spite of the fact that the method according to the invention is very much simpler. Experiences made in mill environment also prove that the difference between the two lastmentioned methods (Curve 2 and 3) is very small. The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the invention idea .

Claims: Claims 1. A method for bleaching lignocellulose-containing fiber material inthe form of chemical pulp without using chloric bleaching agents, at which method the pulp is treated with chelating agent and is bleached with ozone and peroxide, c h a r a c t e r i z e d i n that the pulp immediately after the ozone bleaching is treated with chelating agent in a second chelating step at a pulp concentration of 1-10% for 1-30 minutes, where the temperature is 20-90°C and the pH value 1,5-5, and that the pulp thus treated is washed and dewatered to a concentration of 8-40% prior to the peroxide bleaching. 2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d i n that as chelating agent in the second chelating step DTPA or EDTA is used, which is added in an amount of up to 0,5 kg per ton of pulp. 3. A method as defined in claim 1 or 2, c h a r a c t ¬ e r i z e d i n that the bleaching is carried out to a brightness exceeding 83% ISO.

Inventors: DAHL MAARTEN, NORDEN SOLVEIG, DAHL, MAARTEN, NORDEN, SOLVEIG
Applicants: DAHL MAARTEN, NORDEN SOLVEIG, SUNDS DEFIBRATOR IND AB, DAHL, MAARTEN, NORDEN, SOLVEIG, SUNDS DEFIBRATOR INDUSTRIES AB
Application year: 1994
Main classification: D21C9
IPC codes: D21C9